In the field of membrane blood oxygenators employing a multiplicity of porous hollow fibers, it has long been recognized that gas transfer between the lumens of the hollow fibers and the blood circulating outside the fibers is optimized when the fibers of adjacent layers cross one another. For example, U.S. Pat. No. 3,794,468 (Leonard) discloses winding a single porous hollow fiber around a core in such a way that the hollow fiber along one layer is generally parallel to itself but disposed at an angle to the hollow fiber of the immediately adjacent layers. This provides flow channels for fluid on the outside of the hollow fibers having low resistance and desirable flow characteristics without the use of spacers between the layers, which would undesirably increase the prime volume of the resulting product and surface area that blood contacts.
U.S. Pat. Nos. 4,940,617 and 5,143,312 (Baurmeister '617 and '312) disclose forming a structure comprising two superimposed hollow fiber mats that are then spirally wound to form a multilayer hollow fiber body. Each mat comprises a plurality of generally parallel hollow fibers that are held by solid transverse fibers. The hollow fibers of the two superimposed mats are disposed at an equal but opposite angle (other than perpendicular) to the longitudinal direction of the mat. The result is that the hollow fibers of adjacent layers cross one another to form flow channels without the use of spacers. While the concept of employing a mat has certain manufacturing advantages, one problem with the Baurmeister method and product is that it requires the use of two fiber mats in order to accomplish the result of crossing the hollow fibers of adjacent layers. The mats disclosed in the Baurmeister patents are available from Akzo Nobel Faser AG, Wuppertal, Germany.
An additional restriction or limitation of the prior systems discussed above is that they require the hollow fiber structure to extend completely around a core or otherwise form an unbroken wound structure.